Not applicable.
This invention relates generally to electronic systems that include cabinets, racks, subracks and plug-in modules with printed circuit boards for insertion and removal from said rack or cabinets. Specifically, the invention relates to a method of cooling the electronic components in the subrack to provide high performance and fault tolerance.
Cabinets, enclosures, subracks or card cages (xe2x80x9csubracksxe2x80x9d) are enclosures for electronics which provide a precise framework in which printed circuit boards are supported and guided into connector positions, typically using accurate extrusion technology and hard-tooled end plate design. The subrack not only supports the printed circuit board and related electronic components, but can further provide additional functions such as RFI protection, shock and vibration protection, ESD protection and fire enclosure safety for the electronics of a system.
Subracks are useful in electronic structures because they allow a user to configure usable storage space to suit the application, thus removing the necessity to xe2x80x9ctoolxe2x80x9d each configuration as a custom solution. Configuration features such as xe2x80x9cdivider kitsxe2x80x9d allow a user to provide a variety of heights, widths and depths within a single enclosure size. The frame systems employed in subracks further allow the user to select the orientation of the printed circuit boards from a vertical to a horizontal configuration, or to select a combination of the two within the same frame.
Because subracks provide a wide array of electronics within a relatively small space, subracks must be cooled in order to prevent the electronics from overheating. Typically, fan modules and trays are provided in or on the subrack to provide this function. Conventional fan modules or fan trays utilize axial fans or motorized impellers/blowers to move air axially across the top and/or bottom of the printed circuit boards mounted in the subrack. While these devices provide a relatively effective method for cooling the rack, there are a number of problems associated with typical fan configurations.
For example, a typical fan configuration requires that a baffle be provided around the outside of the fan to avoid recirculation of the air, as shown in FIG. 1. These baffles require a great deal of non-value added space, typically at a premium in a subrack system. Furthermore, most prior art enclosure systems require air to intake in the front and exhaust in the rear, as shown in FIG. 2. This requirement forces air to flow in a S-shaped pattern making 90 degree bends from the bottom to the top, and thermodynamic work is therefore spent to change the direction of airflow. In all cases, irreversible airflow losses occur.
Additionally, problems arise when a fan or other air moving device located directly above or below a set of printed circuit cards fails. When the cooling system fails, the corresponding printed circuit boards or cards can fail in a matter of minutes due to the lack of air movement and because of overheating.
To prevent overheating and failure, one known approach is to provide air coolant redundancy by using a serial or xe2x80x9cpush-pullxe2x80x9d fan configuration. In a push-pull configuration, a first set of fans pushes air into the subrack and a second set of fans pulls air from the subrack. In the event that one fan or set of fans fails, therefore, a second set of fans provides air flow through the rack. The push-pull fan system therefore provides a useful means for cooling subracks. However, push-pull systems also have a number of disadvantages. First, when either the push or the pull fan fails, the failed fan provides an impedance to airflow in the direct path of the opposing fan, and air therefore tends to flow more efficiently through neighboring cards, rather than through the bank of printed circuit boards expected to be cooled by the fan. Furthermore, high flow rates are difficult to achieve with a push pull configuration because, when the fans are installed in series, the maximum volume of air is limited to that of a single fan.
A known alternative to the serial configuration is the parallel configuration. In the parallel configuration, fans are installed parallel or adjacent to each other. Fans installed in this way can provide twice the volume of air flow as a single fan. Parallel fans, however, do not provide the redundancy of push-pull configurations, and further require significant space, which can be problematic in small subrack enclosures.
Neither the serial nor the parallel configuration, furthermore, has been shown to be particularly effective in electronic systems which require a fault tolerant airflow scheme to insure no functionality is lost due to overheating. Mission critical systems and large electronic infrastructure systems, for example, cannot tolerate outages for more than a few seconds per year. In these systems, therefore, a maintenance technician is typically notified automatically when a single fault condition occurs. Despite these precautions, several hours if not days may go by, however, before a fan module can be replaced. In order to meet the expected level of reliability, the cooling system is expected to maintain sufficient air flow through the system until the system can be repaired.
A complete backup airflow system is not practical given the expensive space it would need to occupy and the additional cost of supplying two cooling systems. There remains a need, therefore, for a fault tolerant airflow system which can provide sufficient air flow to allow a system to operate indefinitely with a single fan failure, and further, which is compact and provides sufficient airflow with a high degree of efficiency.
An object of the present invention is to provide a cooling system that is inherently fault tolerant and also highly efficient without the need for fan speed control. The present invention provides a fan module for use in an electronic cabinet, rack or subrack which provides increased air flow as compared to typical prior art fan modules, and assures continuous air flow even when one or more fans has failed. In one aspect of the invention, the fan module comprises a plurality of fans which are successively spaced along a first Cartesian coordinate (i.e. the z direction) and which are staggered or offset along a second (x) and/or third (y) Cartesian coordinate. The offset or staggering of the fans lowers the impedance to airflow through the module in the event of the failure of one or more fans, thereby allowing air flow to continue in the event of a failure. The offset or staggering also provides a consistent air flow through the module, thereby limiting the tendency of air to circle back and re-circulate through a given fan, and minimizing or preventing the need for a baffle around each fan.
In another aspect of the invention, successive fans in the fan module are angled at a successively greater degree above the z direction to direct the air flow through the fan module. The fans can be angled to provide an air flow at any selected angle, where the selected angle depends on the relative positions of the air intake and exhaust for the fan module. For example, an air intake is positioned in a bottom panel of the fan module and an exhaust is located in a side panel of the module. The fans in the module are successively angled between zero and ninety degrees to filly direct air flow from the intake to the exhaust. In this application, the fan closest to the intake is at or near a zero degree angle, while the fan closest to the intake panel is at or near a ninety degree angle. Each of the fans in between are spaced at successively larger angles between zero and ninety degrees. Because the fans are angled rather than directed specifically at one side of the fan module, a number of the fans can be directed at the intake area. For example, the fans can be arranged such that the first and second fans can each draw air through the intake panel. Consequently, a larger intake area can be provided, thereby increasing total air flow through the fan module. Although the fan module has been shown and described for directing air flow at a ninety degree angle, other configurations can also be provided. One typical application provides an intake and an exhaust which are offset by one hundred and eighty degrees.
Preferably, the exhaust of the fan module comprises a metallic honeycomb panel or other cellular, geometric structure capable of allowing sufficient air flow through the structure, but limiting electromagnetic interference by providing a wave guide. In a preferred embodiment, the fan module of the present invention is located on top of a subrack including a card cage. The bends and corners in the subrack enclosure and card cage are constructed with a bend radius greater than the typical ninety degree corner. The larger bend radius minimizes impedance losses to the air flow and therefore maximizes air flow around the corners.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefor, to the claims herein for interpreting the scope of the invention.